1. Technical Field of the Invention
The present invention relates to a circuit board which is produced by flipchip-mounting a semiconductor chip on a base board, and a display device and electronic equipment, employing the circuit board.
2. Description of the Related Art and Problems to be Solved by the Invention
A technology for directly mounting electronic components, such as IC chips, on a FPC (Flexible Printed Circuit) board is currently in increasing use to keep pace with a current trend toward to a smaller, thinner, and lighter board, to meet a bendable design requirement, and to accomplish high yield using a roll-to-roll process. Since such a technology does not need inner leads, like the TAB (Tape Automated Bonding) technology, thin copper foils work, and the promotion of fine pitch design in wiring is easy.
FIG. 16 is a cross-sectional view showing a conventional structure in which an electronic component is mounted on an FPC board. As shown, an input electrode 450a of an IC chip 450 and an input wiring 420a beforehand formed on the base film 410 are electrically connected to each other through electrically conductive particles 21 dispersed within an adhesive resin 19. Similarly, an output electrode 450b of the IC chip 450 and an output wiring 420b beforehand formed on the base film 410 are electrically connected to each other through electrically conductive particles 21 dispersed within an adhesive resin 19.
Since the base film 410, as a base material for the FPC board, typically contains organic film, such as PI (polyimide), which is pervious to water, moisture penetrates from the back side (the underside in FIG. 16) opposite the mounting side into the base film 410 and reaches the wiring formation plane of the IC chip 450. For this reason, the reliability of the IC chip is compromised, or light leakage of light transmitted through the base film 410 degrades the reliability of the IC chip. This disadvantage becomes more pronounced if the base film 410 is made thinner.
Since the polyimide having a low surface wettability is typically used for the base film 410 as the base material of the FPC board, a bonding strength of an adhesive resin is extremely weak.
The effect of thermal coefficients of expansion, different from the base film 410, to the IC chip 450, and to the adhesive resin 19, allows stress to concentrate in a bonding interface with time, and lowers the bonding strength, thereby leading to a poor contact between the electrodes of the IC chip 450 and the wiring of the FPC board. Particularly when the base film is a thin material having flexibility, moisture penetrates from the base film, and the bonding strength drops, thereby leading to a poor contact between the electrodes of the IC chip 450 and the wiring of the FPC board in a more pronounced fashion.
In view of the above problem, the present invention has been developed. The object of the present invention is to provide a circuit board, of a type that has an IC chip, such as a semiconductor chip, mounted on a base board using an adhesive resin, and capable of at least one of the following goals, and a display device and electronic equipment, both employing the circuit board.
1) Improving the bonding strength of the adhesive resin to the base board;
2) Improving electrical connection reliability between the semiconductor chip and a wiring board having the base board and the wiring;
3) Preventing drops in the performance and reliability of the semiconductor chip; and
4) Reducing the penetration of moisture from the back side opposite the mounting side to the bonding area of the base board.
(1) A circuit board of the present invention includes a wiring board including a base material having insulation, and a plurality of board-side terminals arranged on the base material, and a semiconductor chip, mounted on the wiring board, having a plurality of semiconductor-side terminals, with the semiconductor-side terminals electrically connected to the board-side terminals, wherein the wiring board includes a dummy wiring layer, which is isolated from the board-side terminals and the semiconductor-side terminals, and which is inside the board-side terminals, and within an area where the semiconductor chip is mounted.
In accordance with the present invention, at least part of moisture penetrating through the base material from the side opposite the mounting side of the semiconductor chip is blocked by the dummy wiring layer. This arrangement reduces the possibility that a moisture penetration degrades the reliability of the board-side terminals and the semiconductor-side terminals in the area of the wiring board where the semiconductor chip is mounted. When the semiconductor chip is mounted on the wiring board, the circuit board is subject to short circuits and half-short/half-open circuits under the effect of moisture, because the pitches of the board-side terminals, the semiconductor terminals, and a wiring pattern are typically fine. The circuit board of the present invention reduces the creation of such a problem.
A current leakage can take place in the semiconductor chip due to the penetration of light. Since the dummy wiring layer reduces the quantity of light admitted through the base material, a drop in the performance of the semiconductor chip associated with the current leakage is controlled.
(2) A circuit board of the present invention further includes a resin encapsulation section, which encapsulates an area where the semiconductor-side terminals are electrically connected to the board-side terminals, and bonds the semiconductor chip to the wiring board.
In accordance with the present invention, a bonding area of the resin encapsulation section to the wiring board is increased by arranging the dummy wiring area inside the board-side terminals, and within an area of the surface of the base material where the semiconductor chip is mounted. The bonding strength is thus enhanced. As a result, the bonding strength of the resin encapsulation section to the wiring board is increased, and the connection reliability between the semiconductor-side terminals and the board-side terminals is enhanced. Further with this arrangement, the dummy wiring layer prevents the penetration of moisture from the base board, and a drop in the bonding strength of the resin encapsulation section with time is thus controlled. Since the dummy wiring layer is constructed of a material having a higher bond to the resin encapsulation section than to the base material, the bonding strength of the resin encapsulation section to the wiring board is increased.
(3) In a circuit board of the present invention, the resin encapsulation section is manufactured of an anisotropic conductive film which is formed by dispersing electrically conductive particles into a resin, and the semiconductor-side terminals are electrically connected to the board-side terminals by the electrically conductive particles.
If the semiconductor chip is bonded to the wiring board using the anisotropic conductive film in this way, a bonding step and a mold step, corresponding to two steps in the TAB (Tape Automated Bonding) mounting, are carried out in a single step, and the manufacturing process is thus shortened.
(4) In a circuit board of the present invention, the dummy wiring layer is constructed of the same material as the material of the board-side terminal.
When the board-side terminal is manufactured of Cu (copper), for example, the dummy wiring layer is also manufactured of Cu along with the board-side terminal. When the board-side terminal of Cu is plated with Ni or Au, the dummy wiring layer of Cu is also plated with the same material.
The dummy wiring layer and the board-side terminal are thus produced at the same time, thereby simplifying the manufacturing process of the circuit board.
(5) In a circuit board of the present invention, the base material is constructed of a material having flexibility.
For example, the base material is constructed of a material, such as polyimide, having flexibility. When the base material is formed of a thin material having flexibility, the base material is deformable under the application of an external force. For this reason, when the resin encapsulation section bonds the semiconductor chip to the wiring board, stress concentrates in a bonding interface in a localized fashion between the base material and the resin encapsulation section, reducing the bonding strength and thereby leading to a poor contact between the semiconductor-side terminal and the board-side terminal with time. When the base material is a thin material of polyimide, for example, the penetration of moisture from the base material reduces the bonding strength, and thereby easily leads to a poor contact between electrode terminals with time. In a circuit board in which a semiconductor chip is mounted on a wiring board constructed of a base material having flexibility using an adhesive agent, if the dummy wiring layer is arranged inside the board-side terminals and within the area of the surface of the base material where the semiconductor chip is mounted in accordance with the present invention, the bonding strength of the resin encapsulation section to the base material improves, and the connection reliability between the semiconductor-side terminal and the board-side terminal is enhanced.
(6) In a circuit board of the present invention, the base material is directly connected to each of the dummy wiring layer and the board-side terminals without any intervening adhesive layer.
When each of the dummy wiring layer and the board-side terminals is directly connected to the base material without any intervening adhesive layer, the circuit board is free from a current leakage due to the adhesive layer and a swell of the adhesive agent, and the flexibility of the wiring board is increased.
(7) A circuit board of the present invention further includes an opening, which is in contact with the dummy wiring layer in a plan view, and through which the base material is exposed, and which is continuous with the peripheral exposed area of the base material, and the dummy wiring layer is in contact with the opening only, in a plan view.
In accordance with the present invention, when the semiconductor chip is pressed onto the adhesive resin, to become the resin encapsulation section, against the wiring board, the adhesive resin is pushed outwardly to an external area through the opening, residual stress in the resin encapsulation section is reduced. The thickness of the resin encapsulation section is thus made uniform. With this arrangement, the base material is free from distortion and has a reduced residual stress, and the connection reliability is increased.
(8) In a circuit board of the present invention, the dummy wiring layer is a single continuously connected region.
(9) In a circuit board of the present invention, according to (8), the dummy wiring layer is a region curved in a serpentine fashion.
(10) In a circuit board of the present invention, according to (8), the dummy wiring layer is a region that is a combination of a plurality of first line segments mutually running in parallel and a second line segment which extends across the first line segments.
(11) In a circuit board of the present invention, according (1) through (6), the dummy wiring layer includes at least one aperture.
In accordance with the present invention, the dummy wiring layer having the aperture is formed on the base material in the area where the semiconductor chip is to be mounted. When the resin encapsulation section bonds the semiconductor chip to the wiring layer, the bonding interface between the wiring layer and the resin encapsulation section becomes textured. The mechanical engagement between the wiring board and the resin encapsulation section adds to the bonding strength. The reliability of the bond is thus substantially improved.
(12) In a circuit board of the present invention, according to one of (1) through (6), the dummy wiring layer is formed of a plurality of dummy wiring regions which are spaced by exposed areas of the base material.
In accordance with the present invention, when the resin encapsulation section bonds the semiconductor chip to the wiring layer, the bonding interface between the wiring layer and the resin encapsulation section becomes textured. The mechanical engagement between the wiring board and the resin encapsulation section adds to the bonding strength. The reliability of the bond is thus substantially improved.
(13) In a circuit board of the present invention, according to (12), the plurality of the dummy wiring regions are partially connected to each other. With this arrangement, the bond between the resin encapsulation section and the wiring board is enhanced when the resin encapsulation section bonds the semiconductor chip to the wiring board.
(14) In a circuit board of the present invention, according to one of (1) through (6), the dummy wiring layer includes at least one recessed area where the thickness of the dummy wiring layer is thinner than the thickness of the dummy wiring layer on the remaining area.
In accordance with the present invention, when the resin encapsulation section bonds the semiconductor chip to the wiring board, the bonding interface between the wiring layer and the resin encapsulation section becomes textured. The mechanical engagement between the resin encapsulation and the wiring board adds to the bonding strength. The reliability of the bond is thus substantially improved.
(15) A display device of the present invention includes a circuit board according to one of (1) through (14), and a flat panel having connection terminals electrically connected to the circuit board.
(16) In a display device of the present invention, according to (15), the flat panel is a liquid-crystal panel including a pair of opposing substrates, and a liquid crystal interposed between the pair of the substrates, and the connection terminals are formed on at least one of the pair of the substrates.
(17) Electronic equipment of the present invention includes, as display means, a display device according to one of (15) and (16).